Firstly, proper liar lost coordinate (9)
I believe the answer is:
primarily
'firstly' is the definition.
(I've seen this before)
'proper liar lost coordinate' is the wordplay.
'proper' becomes 'prim' (prim and proper).
'lost' indicates anagramming the letters.
'coordinate' becomes 'Y' (as in X and Y coordinates on a grid).
'liar' is an anagram of 'aril'.
'prim'+'aril'+'y'='PRIMARILY'
(Other definitions for primarily that I've seen before include "Chiefly, for the most part" , "Chiefly, in the first place" , "Basically" , "Firstly" , "Chiefly, principally" .)